1. Field of the Invention
The present invention relates to an energy transfer apparatus, and more particularly to an energy transfer apparatus for reducing conductivity electromagnetic interference and a manufacturing method thereof.
2. Description of Related Art
In the PWM technique, much attention is paid to the influence of conductivity electromagnetic interference produced by switching a switch device with a high frequency on electronic devices. In the field of power supply, the conductivity electromagnetic interference can be lowered to meet the requirements of different international safety certifications.
FIG. 1 shows a prior art circuit of a power supply, an input winding NP of the power supply 1 receives an input voltage VIN through an energy transfer apparatus 10. When a controller U1 controls the switching of a switching device Q1, the input terminal of the power supply 1 transmits the stored energy to an output terminal through a core 103. An output voltage VOUT is generated in an output winding NS and a supply voltage VDD is produced in a supply winding NA in response to the turn ratio of an input winding NP (between Node 8 and Node 10), an output winding NS (between Node 1 and Node 3), and a supply winding NA (between Node 7 and Node 5). In the foregoing process of the energy transfer, the input winding NP and the output winding NS will produce conductivity electromagnetic interference due to high switching frequency and the effect of a parasitic capacitor 109 between the input winding NP and the output winding NS.
Referring to FIG. 1, for reducing conductivity electromagnetic interference, the first method of the prior art is to place a shielding capacitor 108 between an input ground terminal of the energy transfer apparatus 10 and an output ground terminal to stabilize both of the electric potential. In practical applications, the shielding capacitor 108 is also known as a capacitor Y. Since the shielding capacitor 108 has a leakage current, the safety is poor and unable to meet the requirements of safety standards. Besides, in the low-power system, the cost is relatively increased for the additional shielding capacitor 108. The second method of the prior art is to place a copper foil having a shielding effect between the input winding NP and the output winding NS for reducing conductivity electromagnetic interference, wherein the copper foil is designed in the energy transfer apparatus 10. Referring to FIG. 2 for the cross-sectional view of a prior art energy transfer apparatus. The copper foil 113 is installed between the input winding NP (between Node 8 and Node 10), the output winding NS (between Node 1 and Node 3), and the supply winding NA (between Node 7 and Node 5) to reduce conductivity electromagnetic interference. When the core 103 is connected to a bobbin (not shown in the figure), the volume and space increased by installing the copper foil 113 will break or damage the core 103 and the bobbin easily.
In recent development, various techniques in wound components have been proposed for power converters to reduce a displacement current flow, such as U.S. Pat. No. 6,549,431 (Odell and Park). The disclosed patent installs a cancellation winding and a balancing winding at the input winding and the output winding respectively. The cancellation winding and the balancing winding utilize a copper wire with a smaller volume to substitute the copper foil to achieve the shielding effect and further reduce the conductivity electromagnetic interference. The cancellation winding and the balancing winding in the aforementioned patent must be the additional windings of the energy transfer apparatus, which will cause insufficient winding space. When the core is connected to the bobbin, the core and the bobbin may be cracked or damaged due to the insufficient winding space. In addition, the cancellation winding and the balancing winding also increase the manufacturing cost. U.S. Pat. No. 6,762,946 (Odell and Park) is a continuation patent of the '431 patent, and U.S. Pat. No. 6,894,909 (Odell and Park) is a continuation patent of the '431 patent and the '946 patent. Furthermore, U.S. Patent Publication US20040246749 (Odell and Jutty) provides a method and an apparatus of installing an additional windings for reducing displacement current flow based on the foregoing '431 patent. In addition to the installation of a secondary balancing winding on the output winding, a cancellation winding and a primary balancing winding are installed on the input winding. Such arrangement also installs a copper wire with a smaller volume to substitute the copper foil to achieve the shielding effect. However, installing more additional windings make worse for the problems of the insufficient space and the increased cost.